The shape of a step structure as a design aspect to control droplet generation in microfluidics

In this paper, silicon-based devices with a step structure integrated at the flow-focusing junction were designed, fabricated and characterized for droplet generation. A two-step silicon etching method was demonstrated to create the step structure. During fabrication, undesirable spikes encountered at the step edge were removed by oxygen plasma ashing and silicon isotropic etching. With this method, two types of step profile (flat and triangular profiles) were fabricated. These two profiles were compared for their differences in droplet-generation behavior. The device with the flat-step profile was found to make larger droplets and at a lower frequency compared to the device with the triangular-step profile. Additionally, polydimethylsiloxane and glass were tested as capping materials for the devices and the impact of their surface characteristics (hydrophobic and hydrophilic) on the type of droplets (water-in-oil or oil-in-water) formed was investigated.